One of the most desired qualities of an image sensor is high sensitivity. A high-sensitive sensor may obtain high-quality images even in a dark state, and may also obtain relatively high-quality images even though a frame rate is increased to capture images of fast-moving objects.
A binning technique is typically adopted for a high-sensitive sensor. Basically, a high-quality and high-sensitive image is obtained by adding values of adjacent pixels according to the binning technique. A binning operation may be implemented as an operation of adding signal values of adjacent pixels by adding accumulated charges in pixels, or may be implemented as an operation of adding analog output values of pixels in a column circuit. Alternatively, the binning operation may be performed in a digital image processing unit.
In general, according to the binning technique using a bayer pattern, by adding or averaging two or more pixels of the same color in a bayer image step, resolution of a screen is decreased, and an SNR is improved.
Hereinafter, a conventional binning technique will be described with reference to the accompanying drawings.
In a 2:1 binning technique typically used in an image sensor using a bayer pattern, a bayer pattern is received, pixels having the same colors are paired in vertical and horizontal directions, and values of the pixels are added or averaged at a ratio of 1:1 to determine values of the pixels, thereby obtaining vertical and horizontal binning images. Then, by using the binning images, a modified image, in which pixels are not uniformly arranged but cluster, is obtained.
Positions of pixels obtained through the above-described process may be compared with those of pixels obtained through ideal sampling assumed in a generally used demosaicing technique. Here, according to the demosaicing technique, the whole RGB colors are restored for the whole pixels from a bayer pattern image in which only one color value from among RGB is assigned to one pixel.
An image obtained through a general scaling technique after applying the demosaicing technique, without performing the binning technique, is excellent in terms of image quality as illustrated in FIG. 3A. However, since a size is reduced after performing an ISP process with an original image, power consumption increases.
In the case of applying the demosaicing technique after performing the binning technique, an inclined edge of an binning image, such as a branch of a tree, is roughly represented, i.e. a jagged edge phenomenon occurs. This phenomenon occurs since sampling positions are irregularly changed due to binning.
To overcome the above-mentioned limitation, a correction filter for correcting sampling positions is used before applying the demosaicing technique after performing binning. As a result, the jagged edge phenomenon is reduced through the correction filter, but resolution of a screen is degraded.